Disposal of nuclear wastes is an important problem in the nuclear energy field today since many radioactive wastes must be stored for very long periods to assure that no health hazard will occur. Low level nuclear combustible solid waste materials are a particular problem because of the relatively large bulk of materials associated with small amounts of contamination. Typical combustible solid waste materials of concern are those resulting from fuel fabrication operations, such as used rubber gloves, paper, rags, metals, glassware, brushes, and various plastic. Of particular concern as well is the disposal of spent ion exchange resins from reactors, fuel fabrication plants, and reprocessing plants, estimated to comprise from 500 to 800 cubic feet of material per year per nuclear reactor.
Present practice consists of packaging these solid waste materials in containers ranging from cardboard boxes lined with plastic bags to steel drums, then burying the packages in pits or trenches. This technique involves difficult and expensive handling of the scrap materials, transporting the packaged materials over roadways and finally storing the materials in monitored repositories or burial grounds. Potential release of contamination to the environment is possible as a result of the rapid decay of the containers, or inadvertent combustion, etc. Moreover, in fuel representing plants and fuel preparation plants, spent ion exchange resins contain significant amounts of plutonium as well as other fission products which may preclude direct burial of these resins, and require monitored retrievable storage.
A large percentage of the contaminated solid waste material is simply light-weight, bulky combustible material. Incineration of nuclear solid waste materials has been studied extensively, but it is subject to poor control of combustion, with attendant off-gas system difficulties and severe corrosion problems, coupled with rather expensive maintenance problems. Mechanical compaction of the solid waste material has also been studied extensively with volume reductions of two to ten-fold being achieved. In general, however, compaction and sorting of nuclear solid waste materials are moderately expensive in that special personnel protection devices are needed over and above normal protective equipment costs. Also, compacted solids are readily dispersible in the environment and can generate gases which under certain circumstances may constitute a safety hazard until properly disposed of in an engineered controlled and monitored area.
Acid digestion volume reduction methods appear to have some advantages over incineration, namely more efficient off-gas handling, and generally better reliability and longevity of essential hardware exposed to radioactive materials. Other advantages include a lack of buildup or accumulation of activity in refractory linings, and no generation of a liquid waste stream requiring further treatment. Combustible waste can be wholly digested with acid to an inert, non-combustible residual fraction. This very high sulfated residue fraction is wet with sulfuric and nitric acids but can be immobilized as a high integrity low leachable and low dispersible glass solid. The method involved removal of the acids, solids milling, desulfation of the residue using carbon fines at 700.degree. to 900.degree. C., and glassification of the desulfated material after adding appropriate glass formers and heating to at least 1050.degree. C. The acid digestion waste treatment combined with a residue immobilization process complete the plant processing cycle. Large volumes of easily dispersible waste solids are converted to a small volume (20% reduction) of non-dispersible product compatible with presently used packaging methods. Conversion of the wastes to a non-leachable, non-dispersible solid is desired in order to provide an added safety factor, and possibly lower ultimate cost, in permanent storage.
For some nuclear waste residues, glass-forming additives such as phosphates or borates and lime or magnesia have been employed to obtain a vitreous nonleachable product with good mechanical strength and thermal conductivity. A major difficulty in these processes which form solids containing fission-product contamination has been the tendency of radio ruthenium to volatize, both during evaporation, and calcination, or fusion. For example, in the absence of control measures, ruthenium is normally volatized to the extent of 20 to 60 percent in calcining at the elevated temperatures, i.e., above 850.degree. C., required for producing a ceramic or above 950.degree. C. for a glassy solid. The volatized ruthenium, in the form of fission-product isotopes, ruthenium 103 and ruthenium 106, represents a substantial portion of the gamma activity of these solutions, and off-gas systems are thus severely contaminated. In some solids-forming processes, the volatilized ruthenium has been collected on silica gel or ferric oxide beds and the loaded beds subsequently combined with the calciner product. This procedure, however, is undesirable because of contamination of process equipment and the additional handling of highly radioactive materials required. Other problems preclude addition of phosphates to acid digestion methods due to the serious corrosion problems caused by them. Minimization of nitric acid concentration, pressure, and temperature in evaporation has been employed to minimize volatilization. These measures, however, have not been fully effective in the preparation of glass-like, non-leachable solids where a temperature of at least about 950.degree. C. is required. Ruthenium off-gas losses of 50% or more are common.